Transmission across cable networks such as those used to distribute data or television services typically consist of approximately 1 GHz of spectrum which comprises many channels spaced 6-8 MHz apart. As consumer demand for bandwidth increases, customer premise systems are beginning to downconvert several channels from different parts of the spectrum in order to receive multiple channels for the purposes of additional data bandwidth or to permit the user to watch and record a multiplicity of channels. These channels are often modulated using a single carrier modulation such as 256 QAM used in DOCSIS, J.83 and DVB-C standards. To accommodate additional bandwidth demand, these systems may move to higher order modulation schemes such as 1024-point QAM, which will impose stringent demands on phase noise at the receiver.
Similarly, orthogonal frequency division multiplexed (OFDM) communications systems have been developed to address problems in high data rate communications systems such as expanding bandwidth in the presence of multipath interference. In an OFDM system, a transmitter receives an incoming data stream and modulates the data on orthogonal frequency domain subcarriers. The modulated subcarriers are then transmitted as an OFDM symbol to a receiver. By dividing the incoming data stream among multiple sub-carriers, the data rate and thus the bandwidth of these individual subcarriers is decreased relative to the bandwidth of the incoming data stream. The resulting increase in the duration of the data symbols associated with each subcarrier can decrease the impact of multipath interference and associated inter-symbol interference (ISI).
In OFDM systems, using phase errors which are common to all subcarriers to suppress phase noise within a single channel is a well-known technique. Typically this technique is inherent to channel equalization performed in OFDM receivers, and allows phase noise on the order of the symbol rate to be removed effectively at the same time that the channel response is equalized.
In contrast, in systems processing multiple channels the phase noise from each subcarrier creates a noise skirt which acts as additive noise to other subcarriers and cannot be removed using this approach. In addition, previously known methods have not been possible in single-carrier systems such as cable transmission systems which use single-carrier QAM physical layers such as are used in DOCSIS and DVB-C J.83 annex A and B standards. These systems typically use a carrier tracking loop to remove close-in phase noise on the order of a few kHz, but are unable to remove phase noise at higher offsets. This phase noise is an important source of performance degradation particularly in modulations with higher-order constellations such as 256 or 1024 QAM. Consequently, there is a need in the art for methods and apparatus for providing enhanced performance in the presence of phase noise in multichannel communication systems such as may be used in cable or other applications.